RGB light emitting diodes (LEDs) are more and more used in numerous lighting applications. Each LED may be controlled individually in intensity and colour by an integrated circuit (IC) via a communication interface. The LEDs may need calibration by means of the IC before use to a given target value.
RGB LEDs are typically Pulse Width Modulation (PWM) driven to set a defined current for each colour channel in order to adjust to certain colour points and colour intensities to obtain a dedicated light output of a given RGB LED. For a given target light output the currents in the different colour channels should not change as this might give rise to a change of the spectrum, resulting to a different colour point and intensity.
For optimal light control a high PWM resolution of at least 18 bits on these currents is needed. On the other hand, the PWM resolution is typically limited to 16 bits. This is because a high PWM resolution also requires a high frequency clock generator. The LED itself is driven in a PWM frequency range of up to 500 Hz, so that the human eye does not see flickering. This would be the case, if lower PWM frequencies were used. As a result, a 16 bit PWM resolution needs a clock source of about 32.768 MHz (65535*500 Hz). Each additional bit increases the required clock signal by factor of two, which may lead to increased electromagnetic emissions and is not cost optimal. Further, the current through the LED needs a certain time to settle, which makes high PWM resolutions useless at low duty cycle values of the PWM. Hence, there is a need for performing light control with a PWM resolution limited to 16 bits.
This problem has been tackled many times in the prior art. For example, WO2015/061237 is concerned with controlling the brightness of a LED display employing a combination of current and PWM dimming of the LEDs. The disclosure offers a solution to deal with the non-linear relationship between the current and the luminous flux, which is applied for dimming the light output in a brightness control.
EP3076758 presents a light regulation loop. A solution for turn on optimization of a driver for one or more light sources is disclosed. A duty cycle value is selected from a table. The selected duty cycle corresponds to the target output current of the driver and has a corresponding voltage. The selected duty cycle is applied to the driver. An output voltage at the light source is measured and compared to the corresponding voltage of the selected duty cycle to produce a voltage comparison result. Based on the comparison result, the selection of the duty cycle is adjusted. Additionally, an output current of the light source is measured and compared to the target output current to produce a current comparison result. An adjustment coefficient is applied to a feedback circuit of the driver based thereon, wherein the feedback circuit adjusts a switching frequency of the driver based on the selected duty cycle.
There is however room for alternatives for performing LED light control over a wide dynamic intensity range with a limited PWM resolution.